This invention pertains to electrode elements for monopolar electrolysis cells having planar, opposed electrode surfaces arranged vertically and substantially parallel to one another, and fastened along with electrode connections to an electrode frame. Such electrolysis cells are especially useful for chlor-alkali electrolysis.
Electrolysis cells of this type are typically useful for chlor-alkali electrolysis wherein chlorine, hydrogen and alkali hydroxides are prepared from aqueous alkali chloride solutions by the application of electrical energy,. Chlorine is also obtained as a by-product of the electrolysis of molten salts used in the manufacture of alkali metals or alkaline earth metals. Cells of this type have also been increasingly used in the electrolytic decomposition of hydrochloric acid, and are becoming more significant in this respect.
Some of these products are manufactured in very large quantities as basic chemicals. In the case of chlor-alkali electrolysis, plants are frequently operated with individual current loop production capacities of 500 to 1,000 tons of chlorine per day. In such plants, current intensities of up to about 500,000 amps are attained. Depending upon the particular process used, larger or smaller numbers of electrolysis cells may be combined into a single circuit.
If an electrical direct current flows through an electrochemical cell having an alkali chloride-containing aqueous electrolyte, chlorine gas is primarily formed at the positive pole or anode, while hydrogen gas and alkali hydroxide form at the negative pole or cathode. Reverse reaction due to mixing of the product should, of course, be prevented. For this purpose, two different processes were initially developed: the so-called mercury process and the diaphragm process.
In the diaphram process, a porous separating wall (diaphragm) separates the anode chamber from the cathode chamber and thus prevents mixing and the undesirable reverse reaction of the products separated at the electrodes.
Recently a third electrolysis process, the so-called membrane cell process, has increasingly come into use. Since dimensionally stable anodes and permselective membranes are now available, the electrolysis cells can be manufactured with a thin separating membrane clamped between flat opposed electrodes.
The successive combination of several electrolysis cells of this type yields a cell block with a filter-press-like structure. These filter-press type electrolysis cells are known, for example, from German Pat. No. 1,054,430 and German Offenlegungsschrift No. 2,222,637, the disclosures of which are incorporated herein by reference, which illustrate the electrolysis of aqueous hydrochloric acid, and from German Offenlegungsschrift No. 2,510,396, directed to chlor-alkali electrolysis, the disclosure of which is also hereby incorporated by reference.
In general, the cell elements are held in supporting frames. With the aid of a suitable pressing device, for example a hydraulic press, a tension bar or individual screws, the cell block is pressed together with gaskets placed between the cell elements to seal them off one another, and pressed together to form a rigid unit containing from about 10 up to, for example, 100 cell elements, and having a corresponding production capacity. Such a unit may, if desired, be mounted on a suitable frame.
The electrolysis filter-press type cells can then be connected in bipolar fashion, as illustrated in U.S. Pat. No. 4,056,458, the disclosure of which is hereby incorporated by reference, or, alternatively, in monopolar fashion. If a bipolar arrangement is employed, the first and last electrodes will each have a current connection with the current flowing in a longitudinal direction through the cell block. In such a circuit, either liquid-tight electrodes, which have different polarities on each of their two sides, are used or, alternately, separating walls are provided for current connection between the opposite electrodes.
In a monopolar arrangement of filter-press type electrolysis cells, each electrode frame typically contains two electrodes of similar polarity, and the electrolysis cell block is typically made up by arranging corresponding anodic and cathodic frames alternately in succession. In this manner, a suitable separating wall, for example a membrane or a diaphragm, is supplied to separate the anode chamber from the cathode chamber formed between adjacent electrode frames. Each electrode has an external current connection, which is suitably connected to the opposite electrode of another electrolysis cell, wherein the electrolysis current flowing into each electrode frame is distributed over the electrode surface, flowing perpendicularly to the electrode surface through the electrolyte gap to the opposite electrode, and finally leaves the corresponding adjacent electrode frame of opposite polarity. All electrodes of the same polarity are preferably connected in parallel. An arrangement of this type is aptly described, for example, in applicant's concurrently filed patent application Ser. No. 39,997 relating to an electrolysis cell system the disclosure of which is hereby incorporated by reference.
To facilitate the introduction of the electrolysis current to the electrode surfaces of an electrode element, it is possible to place a corrugated panel with stamped lugs between the two electrode surfaces of an electrode element. The lugs may be connected to the electrode surfaces, for example, by resistance welding. These lugs provide for the transmission of current between the two electrode surfaces and the corrugated panel since they are raised above the corrugations and form a gas-permeable canal between the corrugations and the back of the electrode surface. This gas-permeable channel is necessary to enable the gas generated at the electrode to flow upward without impediment.
There are technical limits to the increase in performance of the electrolysis cell which can be achieved by means of higher specific current loadings with electrode elements of this type. For example, the cross-section of the corrugated spacing panel between the electrode surfaces cannot be enlarged indefinitely due to the possibility of deformation. In addition, manufacturing the electrically conductive connection of the spacing panel with the electrode frame or with the wall of the electrolysis vessel is rather difficult and expensive.
It is thus a primary object of the present invention to provide an improved electrode element of the type described above which will have a simple structure and will permit a high electrolysis current.
In order to achieve this object, a current supply device of the largest possible cross-section between the two electrode surfaces of an electrode element is desired which is, in addition, electrically connected with the electrode surface only at certain points in order to leave room for the passage of the separated gas and other electrolysis fluids between the power supply point and the electrode surfaces.